For a large number of applications, especially in the area of biomaterials for medical devices, it is necessary to modify the inside walls of long and simultaneously thin tubes made of a dielectric material. Such a process includes cleaning, activation, modification and biological decontamination. Typically these modifications cannot be performed during the production of the materials, while in many fields, and depending on the area of application, the modification must be regularly renewed after the manufacturing process has been carried out. Physical plasmas offer a large number of advantages for this kind of application. The modifications achieved are distributed homogeneously over the surface, are very thin (nm range), are strongly adhering and alter the composition and properties of the basic material only very slightly. The different modifications may be achieved by suitable choice of the process medium and of the physical parameters of the plasma. For cost reasons, and for simple integration into existing process steps, the modifications by a physical plasma should take place as far as possible under normal pressure. Heretofore, however, it has proved to be extremely difficult to generate, under normal pressure, a plasma that is homogeneous over the entire length of the tube for a highly variable range of parameters and large aspect ratio of the tubes. Especially for complex medical devices, such as endoscopes, for example, it is difficult to couple electrical fields from outside the endoscope into the interior of the working channels, in order to ignite a physical plasma therewith. From the user's viewpoint, it is also disadvantageous to introduce electrodes into the working channels in order to couple in the power for the plasma, since the surfaces of the channels could be damaged.